EP0880188B1 - CO-tolerant anode catalyst for PEM fuel cell and its method of manufacturing - Google Patents

Description

The invention relates to a supported platinum catalyst for the anode of a PEM fuel cell with good resistance to Poisoning from carbon monoxide. The catalyst contains the precious metals platinum and ruthenium on a fine, conductive carrier material. It is particularly useful as an anode catalyst of fuel cells with a polymer electrolyte membrane suitable.

In principle, fuel cells are gas-powered batteries, where those from the reaction of hydrogen and oxygen gained energy directly converted into electrical energy becomes. The present invention describes the manufacture of catalysts for fuel cells, in particular the production of supported catalysts on the basis of platinum and platinum alloys for PEM fuel cells (PEM = polymer electrolyte membrane). This type of fuel cell wins because of its high energy density and robustness increasing importance for use in the mobile area, i.e. for use in motor vehicles for electrical traction.

The advantages of a car powered by fuel cells are due to the very low emissions and the high Efficiency compared to conventional internal combustion engines. If hydrogen is used as the fuel gas, the result as the only emission water on the cathode side of the Cell. It is then a so-called ZEV (Zero Emission Vehicle). However, hydrogen is current Time too expensive and creates problems with storage and when refueling vehicles. For this Grund wins the alternative, the hydrogen directly To produce on-board the vehicle by reforming methanol increasingly important. The one stored in the vehicle tank Methanol is added in a steam reforming process 200 - 300 ° C in a hydrogen-rich fuel gas with carbon dioxide and converted carbon monoxide as minor components. After the carbon monoxide has been converted by a shift reaction, Preferential oxidation (PROX) or other cleaning processes this fuel gas is directly on the anode side of the PEM fuel cell fed. The reformate gas exists theoretically from 75 vol.% hydrogen and 25 vol.% carbon dioxide. In practice, however, this gas still contains nitrogen, Oxygen and, depending on the degree of purification, changing amounts of carbon monoxide (up to 1% by volume).

As catalysts on the anode and cathode side of the PEM fuel cells are based on supported catalysts of platinum and platinum alloys used. These exist made of fine precious metal particles on a conductive Base material (mostly soot or graphite) deposited are. The precious metal content is between 10 and 40 % By weight, the proportion of the conductive carrier material accordingly between 60 to 90% by weight. The crystallite size of the Particles measured by X-ray diffractometry (XRD), is approx. 2 to 10 nm.

Conventional platinum catalysts are against poisoning carbon monoxide very sensitive, therefore the CO content of the fuel gas to <10 ppm to reduce performance of fuel cells due to poisoning to prevent the anode catalyst. This is especially true for the PEM fuel cell with its low Working temperatures from 70 to 100 ° C particularly sensitive against CO poisoning.

The present invention is concerned with manufacturing supported catalysts based on platinum and ruthenium, which is highly resistant to poisoning Have carbon monoxide. CO contents of over 100 ppm in the reformate gas should be possible and with no noticeable loss of performance of the PEM fuel cell. Through the Use of such novel catalysts on the anode side the PEM fuel cell can be the number of necessary Process steps to remove carbon monoxide from the Fuel gas can be reduced. This leads to a significant one Lower system costs, to improve the System efficiency as well as a downsizing of the overall system. The new catalysts are therefore large Significance for the introduction of the PEM fuel cell in mobile area.

The problem of poisoning by platinum catalysts Carbon monoxide has been known for a long time. CO is due its special molecular structure on the surface of the Platinum adsorbs and blocks the access of the hydrogen molecules of the fuel gas to the catalytically active Platinum centers.

By adding water, the adsorbed carbon monoxide are oxidized to carbon dioxide and can then by the Remove the catalyst surface. It is also known that Platinum catalyst tolerance to poisoning by carbon monoxide by alloying or doping the platinum to improve with ruthenium.

L. W. Niedersach et.al. (J. Electrochemical Techn. 5, 1967, S.318) describe the use of Pt / Ru catalysts as CO-tolerant anode catalysts for sulfuric acid Fuel cells. These materials are made of fine Pt / Ru alloy powders with high specific surfaces. You will learn about the so-called ADAMS process in one Melt from platinum chloride, ruthenium and sodium nitrate 500 ° C manufactured. Due to the high temperatures at the These catalysts are manufactured as Pt / Ru alloys in front. The materials are not fixed on a carrier and are therefore not supported catalysts. Also there is no mention of their use in PEM fuel cells Information before.

Pt / Ru supported catalysts have also been commercial for some time available. The company ETEK, Inc., Natick, Massachusetts (USA) appropriate materials for use as anode catalysts in PEM fuel cells.

It is a Pt / Ru alloy catalyst with precious metal loads between 5 and 40 wt .-% and a Pt / Ru atomic ratio of 1: 1. This catalyst exhibits a uniform alloy phase that can be verified by XRD on. However, it shows an unsatisfactory tolerance against carbon monoxide, especially at concentrations of carbon monoxide over 100 ppm and oxygen residues in the fuel gas.

A recent work by M. Iwase and S. Kawatsu reports on the development of a CO-tolerant anode catalyst (M. Iwase and S. Kawatsu, Electrochemical Society Proceedings, Volume 95-23, p. 12). In this work, the best results were achieved with a Pt / Ru alloy catalyst, which was produced using a special annealing process for alloy formation. The voltage drop at a current density of 0.4 A / cm ² was still approximately 200 mV with a CO content of 100 ppm. This is still too high for practical operation. In contrast, even worse results were achieved with an unalloyed Pt / Ru system, so that based on this information it must be assumed that only alloyed Pt / Ru supported catalysts give the best results with regard to the CO tolerance in the PEM fuel cell.

It is an object of the present invention to support catalysts to provide improved tolerance towards Carbon monoxide, especially at levels above 100 ppm, exhibit. The catalysts are intended for operation with fuel gases containing carbon monoxide, nitrogen and oxygen be suitable and the lowest possible voltage drop show at high current densities.

The task is solved by a platinum supported catalyst for the anode of a PEM fuel cell with good resistance against poisoning by carbon monoxide, containing the precious metals platinum and ruthenium on a fine, conductive carrier material. This is the catalyst characterized in that the two precious metals are not together are alloyed. Rather, they are highly dispersed Form on the support material, the crystallite size of platinum less than 2 nm and that of ruthenium less than 1 nm is.

Surprisingly, it has been shown that these inventive non-alloyed Pt / Ru supported catalysts that through a special alloy that suppresses the formation of alloys Processes are produced with a very good CO tolerance for concentrations up to 150 ppm CO.

The reasons for the improved CO tolerance of the catalyst are not yet fully understood. A possible Explanation is that the speed-determining The CO oxidation step does not react with the Oxygen of the Ru surface, but diffusion of the CO is on the catalyst surface. If the distance of the Ptund Ru crystallite is small, so a high dispersion If both metals are present, the diffusion of the CO can occur quickly respectively. The oxidation properties of the catalyst are positively influenced by it.

This occurs when the two metals form an alloy to an exchange of the lattice sites, which makes a part of the ruthenium no longer on the particle surface Available.

The process for the preparation of the Pt / Ru supported catalysts according to the invention is specially designed for one Prevent alloying of the precious metals and at the same time to achieve a high dispersion.

For the deposition of platinum and ruthenium on the carrier material it is first suspended in water. To this Suspension is added to aqueous solutions of the precursor compounds adding platinum and / or ruthenium to the precious metals and sets the pH of the suspension from carrier material and Precious metal solution by adding lye to a value between 7 and 9 a. It also changes the temperature of the suspension before or after adding the precious metal compounds raised a constant value between 50 and 80 ° C. Subsequently become platinum and / or ruthenium by reduction with a reducing agent completely on the carrier material deposited, the catalyst thus obtained is filtered off, washed and dried.

A high-temperature annealing process as it is used to produce Alloys is avoided. As tempering and drying has vacuum drying at temperatures up to max. Proven at 200 ° C.

The two precious metals can either be simultaneous or in any Sequence in sequence on the carrier material be deposited. Was a sequential deposition is selected, the second precious metal is dried before the Catalyst in the same way as the first precious metal deposited on the carrier material.

An aldehyde group-containing is preferred as the reducing agent Reducing agents such as formaldehyde or sodium formate used.

Carbon black, graphitized carbon black, graphite or activated carbon with specific surfaces (BET) of about 40 to 1500 m ² / g are used as the conductive carrier material. The noble metals are separated by chemical reduction of the corresponding platinum and ruthenium salts from aqueous solution. Chlorine-containing starting compounds such as hexachloroplatinic acid and ruthenium chloride and chlorine-free compounds, for example platinum nitrate, platinum sulfite acid or ruthenium nitrosyl nitrate, can be used. The proportion of platinum and ruthenium is between 10 and 40% by weight, that of the conductive carrier material between 60 and 90% by weight. The atomic ratio of platinum / ruthenium is between 1: 4 and 4: 1, preferably between 1: 1 and 2: 1.

Figur 1:

Gasdiffusionselektrode aus poröser Katalysatorschicht auf hydrophobiertem, leitfähigen Substratmaterial

Figur 2:

Mit Katalysator beschichtete, protonenleitende Polymermembran.

Figur 3:

Membran-Elektroden-Einheit für PEM-Brennstoffzellen.

The catalyst according to the invention can be used to produce various components for PEM fuel cells. Figures 1 to 3 show some examples:

Figure 1 :

Gas diffusion electrode made of porous catalyst layer on hydrophobic, conductive substrate material

Figure 2 :

Proton-conducting polymer membrane coated with catalyst.

Figure 3 :

Membrane electrode unit for PEM fuel cells.

On the one hand, it is possible to use so-called gas diffusion electrodes produce that contain the catalyst (see Figure 1). Such electrodes consist of a hydrophobized, conductive substrate material (2) (for Example hydrophobic carbon paper) on which a porous Catalyst layer (1) is applied.

With the help of these gas diffusion electrodes, the in Figure 3 shown membrane electrode assemblies for PEM fuel cells be built up by the polymer membrane (4) on both sides with such gas diffusion electrodes is brought into contact. For the anode side used a gas diffusion electrode that contains anode catalyst (1) according to the invention. The Counter electrode contains the cathode catalyst (3).

Alternatively, the polymer membrane can also be used without the Intermediate step over the production of a separate gas diffusion electrode can be covered with gas diffusion electrodes. In a first step, as in FIG shown the polymer membrane (4) on both sides with the Provide catalyst layers (1,3), one of which contains anode catalyst (1) according to the invention. By Contacting the catalyst layers with hydrophobized Carbon paper becomes a complete membrane electrode assembly.

The catalysts of the following examples according to the invention were characterized by X-ray spectroscopy (XRD) and analysis. Then they became a gas diffusion electrode and a membrane electrode assembly (MEE) processed, the catalysts on the anode side of the MEE were used.

The CO tolerance was determined in a PEM fuel cell with a cell format of 25 cm ² . A simulated methanol reformate gas with the composition 50-60 vol.% Hydrogen, 10-15 vol.% Nitrogen, 20-25 vol.% Carbon dioxide and 0-5 vol.% Oxygen was used as the anode fuel gas. The voltage drop ΔU (mV), which occurs after the addition of a certain amount of CO, is a measure of the CO tolerance of the catalytic converter. The smaller this voltage drop, the better the CO tolerance of the catalytic converter. The catalysts of the invention generally show ΔU values which are up to 50% better than the comparative values of the commercially available catalyst.

The following examples are intended to explain the invention in more detail.

Example 1: Pt / Ru supported catalyst according to the invention

To a suspension of 81.1 g of Vulcan XC 72 carbon black (Residual moisture content 1.39% by weight) in 2000 ml of deionized Water is added with good stirring at room temperature a solution of 52.7 g of hexachloroplatinic acid from 10 min (25 wt% Pt) and 48.4 g ruthenium (III) chloride solution (14 wt% Ru) in 200 ml deionized water. One warms up to 80 ° C and with sodium hydroxide a pH of 8.5 a. After adding 27.2 ml of an aqueous formaldehyde solution (37% by weight) is filtered off, the moist filter cake washed with 2000 ml deionized water and at Dried 80 ° C in a vacuum drying cabinet.

The analytical data of the catalyst are: Pt content 13.18% by weight Ru content 6.82% by weight Atomic ratio Pt / Ru 1: 1

The catalyst was characterized by XRD. You get the (110) reflection of the platinum at about 2 theta = 40 °. A shift in the reflex due to alloy formation could not be recognized. Against it the (111) reflex of the ruthenium at 2Theta = 44 ° clearly visible.

The crystallite size of the platinum is about 1.5 nm of the ruthenium below 1 nm.

The catalyst is processed into an ink using a solution of NAFION® and applied in this form to a conductive, hydrophobized carbon paper (TORAY, TGC 90). The occupancy is 0.16 mg precious metal per cm ² . The anode thus produced is hot-pressed together with an ion-conductive membrane (Nafion® 117) and a cathode electrode (coating 0.3 mg Pt / cm ² ), and a membrane electrode assembly (MEE) is thus produced.

The measurement is carried out in a PEM single cell (pressureless operation, temperature 75 ° C.), a current density of 0.5 A / cm ^{2 being} set.

The voltage drop ΔU, which after the addition of 100 or 120 ppm CO to the fuel gas occurs, is used as a measure of CO tolerance of the catalyst used.

Result:

Fuel gas composition: 58 vol% H ₂ ; 15 vol.% N ₂ 24 vol% CO ₂ , 3 vol% O ₂ CO concentration: 100 ppm Voltage drop (ΔU) 41 mV CO concentration: 120 ppm Voltage drop (ΔU) 72 mV

The values for the voltage drop ΔU are around the Factor 2 lower than in the comparative catalyst of comparative example 1. This shows the improved CO tolerance of the catalyst.

Example 2: Pt / Ru supported catalyst according to the invention

To a suspension of 80.6 g Vulcan XC72 (residual moisture 0.8 % By weight) in 2000 ml of deionized water are added at room temperature a solution within 10 min with good stirring of 43.2 g platinum nitrate (30.5 wt% Pt) and 34.1 g Ruthenium nitrosyl nitrate solution (20% by weight Ru) in 200 ml deionized water. The mixture is heated to 80 ° C. and the mixture is added Sodium hydroxide a pH of 8.5. After adding 27.2 ml of an aqueous formaldehyde solution (37% by weight) is filtered off, the moist filter cake with 2000 ml deionized Washed water and the catalyst at 100 ° C in a vacuum dried.

Analytical data:

Pt content: 13.18% by weight Ru content: 6.82% by weight Pt / Ru atomic ratio: 1: 1 Pt crystallite size (XRD) <1.5 nm Ru crystallite size (XRD) <1 nm

Here too, the X-ray analysis of the catalyst shows that Existence of an unalloyed system.

As described in Example 1, the catalyst becomes a Gas diffusion electrode and a membrane electrode assembly processed and stored in a PEM fuel cell measured identical conditions. The composition of the Fuel gas corresponds to example 1.

Results:

CO concentration: 100 ppm Voltage drop (Δ U) 40 mV CO concentration: 120 ppm Voltage drop (Δ U) 67 mV

This also shows the improved CO tolerance of the invention Catalyst compared to the comparative example 1.

Example 3: Pt / Ru supported catalyst according to the invention

To a suspension of 40.65 g Vulcan XC72 (residual moisture 1.6% by weight) in 1500 ml of deionized water are added 80 ° C a solution of 26.5 g of platinum nitrate solution (30% by weight) in 100 ml deionized water and then provides with Sodium hydroxide a pH of 8. After adding 10.8 ml of an aqueous formaldehyde solution (37% by weight) is filtered off and the moist filter cake with 3000 ml deionized Washed water.

The moist catalyst is resuspended in 1000 ml deionized water and at room temperature gives 30 g of ruthenium nitrosyl nitrate solution (6.86 wt% Ru) in 100 ml deionized Add water. After heating to 80 ° C with sodium hydroxide solution a pH of 7 is set. After the reaction is filtered off, the moist filter cake with 1000 ml deionized water and dried at 80 ° C in a vacuum.

Analytical data:

Pt content: 15.9% by weight Ru content: 4.1% by weight Pt / Ru atomic ratio: 2: 1 Pt crystallite size (XRD) 1.8 nm Ru crystallite size (XRD) <1 nm Based on the X-ray analysis, there is an unalloyed Pt / Ru system. As in the previous examples, the catalyst is processed into an MEE and examined for its CO tolerance in a PEM fuel cell.

Results:

CO concentration: 100 ppm Voltage drop (ΔU) 45 mV CO concentration: 120 ppm Voltage drop (ΔU) 89 mV

The improved CO tolerance is also shown here in comparison with comparative example 1.

Comparative Example 1:

A commercially available one is used for comparison tests Pt / Ru supported catalyst (EM content 20% by weight, Pt / Ru atomic ratio 1: 1). It represents the stand of technology in this area.

X-ray analysis (XRD) on this material clearly proves this Existence of an alloyed Pt / Ru system. You get one Shift in the Pt (111) reflex, suggesting a solid solution from Ru in Pt indicates corresponding reflexes of pure Ru are not present. The crystallite size (XRD) of the Pt / Ru crystallites are at 2.7 nm.

The catalyst is processed into an ink using a solution of NAFION® and applied in this form to a conductive, hydrophobized carbon paper. The occupancy is 0.18 mg precious metal / cm ² .

This electrode is then pressed together as an anode with an ion-conductive membrane (NAFION® 117) and a cathode electrode (occupancy 0.3 mgPt / cm ² ), thus producing a membrane electrode assembly (MEE). The measurement is carried out in a PEM single cell (pressureless operation, temperature 75 ° C.), a current density of 0.5 A / cm ^{2 being} set.

Results:

Fuel gas composition: 57 vol% H ₂ , 15 vol% N ₂ , 25 vol% CO ₂ , 3 vol% O ₂ CO concentration: 100 ppm Voltage drop (ΔU) 80 mV CO concentration: 120 ppm Voltage drop (ΔU) 128 mV

The values for the voltage drop when adding CO are about a factor of two above the values of the invention Catalysts. especially at CO concentrations above 100 ppm shows the superiority of the new catalysts.

Claims (10)

1. Platinum supported catalyst for the anode of a PEM fuel cell with high resistance to poisoning by carbon monoxide, containing the noble metals platinum and ruthenium on a finely divided, conductive support material, characterised in that the two noble metals are not alloyed with each other and are present in highly dispersed form on the support material, wherein the crystallite size of the platinum is less than 2 nm and that of the ruthenium is less than 1 nm.
2. Supported catalyst according to Claim 1, characterised in that the atomic ratio of platinum to ruthenium is between 1:4 and 4:1.
3. Supported catalyst according to Claim 1, characterised in that the conductive support material consists of carbon black, graphitised carbon black, graphite or active carbon.
4. Supported catalyst according to Claim 1, characterised in that the proportion of platinum and ruthenium is between 10 and 40 wt.%, and proportion of conductive support material is between 60 and 90 wt.%.
5. Process for preparing a supported catalyst according to Claim 1, characterised in that the electrically conductive support material is suspended in water, then aqueous solutions of soluble compounds of the noble metals platinum and/or ruthenium are added to the suspension, the pH value of the suspension is raised to 7 to 9 by adding an alkaline solution, then all of the platinum and/or ruthenium is deposited onto the support material by reducing with a reducing agent, the catalyst obtained in this way is filtered and washed and optionally the second noble metal is deposited in the same manner and then the platinum/ruthenium catalyst is dried at a temperature of not more than 200°C.
6. Process according to Claim 5, characterised in that the temperature of the suspension, before or after addition of the noble metal compounds, is raised to a constant temperature between 50 and 80°C and deposition of the noble metals onto the support material is performed at this temperature.
7. Process according to Claim 6, characterised in that an aldehyde group-containing reducing agent is used.
8. Gas diffusion electrode for the anode side of a PEM fuel cell containing a porous catalyst layer on a water-repellent, conductive substrate, characterised in that it contains the platinum supported catalyst according to Claim 1.
9. A proton-conducting polymer membrane for PEM fuel cells coated with a catalyst, characterised in that the catalyst layer on the anode side contains the platinum supported catalyst according to Claim 1.
10. Membrane electrode assembly for PEM fuel cells which contains a proton-conducting polymer membrane and has gas diffusion electrodes applied to both the cathode and anode sides, characterised in that the catalyst layer on the anode side contains the platinum supported catalyst according to Claim 1.

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